Question

Why does alkyne and alkene undergo addition reaction whereas alkane does not?

Answer 1

Alkanes are already bonded strongly, while alkenes and alkynes have weak `pi`-bonds. They want to undergo addition reactions to turn `pi`-bonds to stronger `sigma`'s and become more stable.

Explanation:

Alkenes and alkynes are unsaturated - they have `pi`-bonds, so don't have the full number of hydrogen that they could have.

This means that they are more unstable than alkanes, since `pi`-bonds aren't as strong as `sigma`-bonds. The alkenes and alkynes want to form more `sigma`-bonds and have a structure more like an alkane, so they undergo addition reactions.

Addition reactions are where more atoms are added to the molecule, not swapped or taken away. This means that the `pi`-bonds have to be taken away and used as `sigma`-bonds with the new atoms, rather than the `sigma`-bonds already there being reattached - it's easier to break `pi` than `sigma`.

Alkanes do not undergo this reaction because they already only have single `sigma`-bonds, and so they cannot become more stable or stronger structurally - they are already at the peak, and so can only swap things around in substitution reactions.